1. Field of Invention
The present disclosure relates in general to valve actuators, and more particularly to a device to assist an actuator in closing a valve.
2. Description of Prior Art
A gate valve is a valve having a body and a bore through the body. A gate is positioned transverse to the body, and moves linearly to obstruct flow through the bore or allow flow through the bore. Gates typically have an aperture that aligns with the bore to allow flow. The gate is sometimes moved, or actuated, by a valve actuator.
The valve actuator can be pneumatic, diaphragm, piston, or hydraulic type actuator that urges a stem downward toward the valve to open the valve. A bonnet is usually connected to the valve, with the stem extending through the bonnet, and then the actuator is connected to the bonnet. Over time, valves may develop excessive gate drag, making it impossible for the actuator to move upward and fully close the valve. Silt and debris can become trapped in a valve, as well as gate and seal interface damage or excessive wear making it difficult or impossible to fully close. There may be times when an actuator does not have enough bonnet stem thrust or spring force to fully close a valve due to changing field conditions.
During oil and gas production, wirelines or tubular members are often lowered through some type of gate valve. Ideally, the wireline or tubular members is removed from the well before the gate valve is closed. However, in some situations wireline or tubular member removal prior to valve closure is not possible and the gate valve is closed on the wireline or tubular member. Some actuators can provide sufficient force to cut wirelines or tubular members. However, not all valves are designed to do so and in an emergency situation, a wireline or tubular member running through the valve may need to be cut to close the valve. The safety of field operations can be jeopardized when a valve is left in a semi-open position because the actuator can not apply the force required to close the valve completely.
Prior art solutions for non-production applications have used double acting actuators. However these are complicated and costly mechanisms. Some production applications require that the actuator will operate under extreme drilling conditions, and then seamlessly transition into less stringent production applications. For certain production applications a mechanical single acting fail safe close actuator is required and a double acting is not allowed. Production actuator design is generally less robust than the drilling actuator design.